DE4132584C2 - Electrolyte / electrode arrangement for a solid electrolyte fuel cell - Google Patents

Description

The invention relates to an electrolyte / electrode arrangement, that in a solid electrolyte fuel cell is used, and in particular such an arrangement of the so-called substrate structure, in which a thin, flat Solid electrolyte element and thin, flat electrodes be applied to a thick, porous substrate (carrier).

Solid electrolyte fuel cells are fuel cells who use a solid electrolyte element, for example made of zirconium dioxide, so that the fuel cell at high temperatures, for example in the range of about 800 to about 1000 ° C, can be operated. Compared to other known ones Types of fuel cells that occur in solid electrolyte Fuel cell no problems due to retention and corrosion of the electrolyte, and there is no need for a catalyst for reduction any activation surge during operation.

Two types of planar fuel cells have been developed. One has a conventional thin three-layer structure (Anode / electrolyte / cathode), which is the self-supporting Structure calls. The other is one new type of substrate called a structure, in which a thin flat solid electrolyte element and a thin, flat one Cathode applied to a thick, porous anode substrate will. The self-supporting structure is characterized by a  higher energy density than the substrate structure, because at the latter the thick substrate prevents gas diffusion, which limits the current density. The thin electrode of the self-supporting structure does not hinder gas diffusion. However, their thin structure is mechanically so weak that it no large cell areas allowed and the maximum electrolyte plate size to about 20 × 20 cm with a thickness of 0.2 to 0.3 mm is limited. Therefore, the self-supporting Building on applications in the military or in the Space limited where small, compact energy sources high energy density are required.

The substrate structure, however, allows the production of large Electrolyte plates because the thick anode is large but thin Can carry electrolyte plates. Electrolyte plates up to 40 × 40 cm with 2 to 3 mm thick anodes are possible, albeit the energy density for the reasons mentioned is less than that of similar self-supporting structures. Large electrolyte plates therefore make it possible to use fuel cells large capacity to build what the substrate structure the use in power plants for central or decentralized Energy supply has opened. Still stay in this Respect to solve some problems.

Fig. 2 is an exploded view of a conventional electrolyte / electrode assembly of a fuel cell of the substrate structure. In this arrangement, a thin, flat electrolyte element 2 and a thin, flat cathode 3 are applied to a thick anode substrate 1 . An alternative design using a thick, porous cathode as the cathode substrate is possible if the porous cathode has sufficient mechanical strength. The anode substrate 1 , which also serves as the anode, is a porous substrate with ribs and consists of an electrolyte such as zirconium dioxide. An electrically conductive material made of nickel or a nickel-zirconium dioxide metal ceramic is embedded in the porous substrate in order to give it electrical conductivity in its thickness direction. The cathode 3 consists of lanthanum manganite, LaMnO₃. For the solid electrolyte element 2 , zirconium dioxide YSZ stabilizing with yttrium oxide is usually used.

The electrolyte / electrode arrangement thus obtained is assembled as shown in FIG. 2 to form a unit cell with a ribbed connecting part. The connecting part is formed by a cathode substrate 4 made of LaMnO₃ and a separator 5 made of La (Ca) CrO₃. The separator 5 is designed as a calcium-doped layer on the cathode substrate 4 . A fuel cell stack or block is produced in a known manner by alternately arranging the electrolyte / electrode arrangement and the separator and attaching fuel and air distributors to the sides of the stack.

Conventionally, the anode substrate 1 and the cathode substrate 4 are made of nickel oxide-zirconia (NiO-YSZ) powder and lanthanum manganite (LaMnO₃) powder as raw materials for the respective parts. Your parts are formed by pelletizing, layering, extrusion molding or cold isostatic pressing and by sintering in an oxidizing or reducing atmosphere. Usually, the NiO-YSZ anode substrate within a fuel cell is reduced during operation by a fuel gas flow.

The NiO-YSZ anode substrate undergoes a 5 to 6% volume reduction due to this reduction. This contraction is observed even when the Ni content is reduced to approximately 30% by volume, at which point the electrical conductivity is ensured during the reduction. When a dense YSZ solid electrolyte layer is formed on this NiO-YSZ anode substrate, there arises a problem that the YSZ layer cracks due to the change in the NiO volume during the reduction, which eventually leads to warpage and cracks in the electrode substrate itself to lead. In the case of a NiO-YSZ anode substrate with a diameter of at least 100 mm, cracks also occur in the substrate itself simply by the formation of the YSZ layer. This indicates that the porous anode substrate is unable to measure the difference between the thermal expansion coefficient of the NiO-YSZ of (12 to 14) × 10 ^-6 / ° C (30-1000 ° C in air) and the thermal Expansion coefficient of the solid electrolyte element YSZ of 10.5 × 10 ^-6 / ° C to absorb.

The above-mentioned volume reduction or contraction also shows up when the cathode is used as the substrate for the electrolyte. The LaMnO₃ cathode substrate also developed a strong volume reduction. If a dense YSZ layer is formed on this LaMnO₃ cathode substrate, cracks develop in the YSZ solid electrolyte layer due to the change in the LaMnO₃ volume, which ultimately lead to curvature and cracks in the electrode substrate itself. This shows that the porous cathode substrate is unable to differentiate between the thermal expansion coefficient of LaMnO₃ of 12 × 10 ^-6 / ° C (30-1000 ° C in air) and the thermal expansion coefficient of the solid electrolyte element YSZ of 10.5 × 10 ^-6 / ° C to absorb.

EP-A 0 105 592 relates to an electrochemical power generator in which a pair of flat electrodes (which are referred to in FIG. 2 of the document as "25" and "29") are arranged at a distance from one another and a solid electrolyte element ("26" ) is sandwiched between the electrodes. Both the anode material and the cathode material are porous materials. However, the respective electrode catalyst layers made of a metal can hardly be described as an electroconductive material as "embedded in the porous material" as is the case in the present invention.

Document US-A 4 950 562 discloses fuel electrolyte cells in which zirconium dioxide stabilized with yttrium is used as the electrolyte material, as cathode material for example, lanthanum strontium manganite is used, and as an anode material a Ni-ZrO₂ cermet is used, as already in the prior art was known before this document. The thermal expansion coefficients of each Portions of the fuel cell should follow the teachings disclosed in US-A 4,950,562 are as close as possible to one another to make an improved and more reliable fuel cell to accomplish. This is achieved according to this document in that the so-called "interconnector" is coated with a layer made of a composite metal oxide of the Perovskite type exists.

The publication US-A 4 562 124 is also with the adaptation of the thermal expansion coefficient of the electrode material to that of the other components of a solid electrolyte fuel cell deals. The in relation to the coefficient of thermal expansion the thermal expansion coefficients of the other components of the fuel cell however, adapted materials are the cathode materials. From column 1, rows 51 ff., As well as column 2, lines 53 ff. And claim 1 of the document, it follows that the material of the cathode is lanthanum manganite or lanthanum chromite, in which a small Amount of lanthanum is replaced by cerium. Replacing the lanthanum with cerium reduces the thermal expansion coefficient, so that it matches the thermal expansion coefficient of the carrier or the electrolyte material.

The object of the present invention is to achieve the above-mentioned Solving problems and an electrolyte / electrode arrangement without creating cracks in that an electrode substrate is formed, the thermal with the solid electrolyte element is compatible in the unit. Task of Another invention is an electrolyte / electrode arrangement to create in the solid electrolyte element and porous matrix to avoid jumps, components with essentially the same thermal expansion coefficient exhibit.  

According to the invention, this object is achieved by an electrolyte / electrode Arrangement solved according to claim 1.

Advantageous developments of the invention are in the subclaims 2 to 6 marked.

By optimizing the composition of the electrode substrate, so this at least one same chemical Component like the solid electrolyte and a Contains electrically conductive material, the electrode substrate thermally to the solid electrolyte element in the unit cell to be adjusted while the conductivity of the electrode substrate is increased.

Exemplary embodiments of the invention are described below the drawings explained in more detail. Show it:

Fig. 1 is a perspective exploded view of an electrolyte / electrode assembly of a fuel cell with a substrate structure according to an embodiment of the invention;

Fig. 2 is an exploded perspective view of a conventional electrolyte / electrode assembly of a fuel cell with a substrate assembly;

FIG. 3 shows the crystal structure of the anode substrate according to an embodiment of the invention, taken with a scanning electron microscope;

Fig. 4 is a graph showing the relationship between the amount of added coarse YSZ or MSZ powder to the anode substrate and the thermal expansion coefficient of the anode substrate; and

Fig. 5 is a graph showing the relationship between the amount of added coarse YSZ or MSZ powder to the anode substrate and the resistivity of the anode substrate.

Fig. 1 shows an exploded perspective view of an electrolyte / electrode assembly for a fuel cell with a substrate structure according to an embodiment of the present invention. In this embodiment, a flat anode substrate 7 was manufactured by machining internal ribs on one side of a 4 mm thick porous NiO-YSZ substrate obtained by cold pressing and subsequent sintering in a manner explained later. The porous substrate consists of zirconium dioxide, which is either partially or completely stabilized with yttrium oxide or magnesium oxide, and electrical conductors made of nickel-zirconium dioxide-metal-ceramic, which are embedded in the porous matrix. The electrically conductive material imparts electrical conductivity to the porous substrate in the thickness direction of the anode substrate. A flat anode 6 is formed on the flat side of the anode substrate 7 . The anode 6 is composed of nickel oxide, NiO, and zirconium dioxide, YSZ, stabilized with 8 mol% yttrium oxide, in a weight ratio of 2: 1, and polyvinyl butyral, PVB, as a binder. The PVB is dissolved in ethanol and the ingredients are mixed wet. The slurry thus obtained is applied to the anode substrate 7 and sintered at 1400 ° C. to form the flat anode 6 on the anode substrate 7 . YSZ is continuously applied to the anode 6 by plasma spraying under reduced pressure to form a solid electrolyte element 8 . A flat cathode 9 is formed by applying a lanthanum manganite coating, LaMnO₃ and subsequent sintering at 1200 ° C. A cathode substrate 10 is made by pressing and sintering the same LaMnO₃ used for the cathode. Its porosity is 30% and the average pore size is 3 µm.

The electrolyte / electrode arrangement thus obtained is assembled as shown in FIG. 1 with a ribbed connecting part to form a unit cell. The cathode substrate 10 is covered with a separator 11 containing lanthanum calcium chromite, La _0.7 Ca _0.3 CrO₃, which is formed under reduced pressure by plasma spraying over the porous matrix of the cathode substrate 10 . A fuel cell stack is produced by alternately stacking an electrolyte / electrode arrangement and a connecting part and attaching fuel and air distributors to the sides of the stack in a known manner.

Manufacturing method for the electrode substrate according to the Invention will hereinafter be described in detail by means of examples explained.

example 1

First, the following coarse zirconia powder is used as the matrix substrate prepares: stabilized with 8 mol% yttrium oxide Zirconium dioxide YSZ is granulated using a spray dryer and provisionally in the air for two hours 1500 ° C sintered. It then becomes a coarse zirconia powder pulverized and passed through a sieve with a Given opening size of 300 microns. The average The grain size of the coarse powder obtained is within in a range from 50 to 100 µm.  

Then a rough electrically conductive material like follows granulated: NiO and stabilized with 8 mol% yttrium oxide Zirconium dioxide YSZ are in a weight ratio of 2: 1 weighed and wet in a solution of polyvinyl butyral, PVB, and polyethylene glycol (PEG) as binders in ethanol mixed in. The coarse YSZ powder mentioned above becomes the Added mixture, mixed this further wet, left standing, and then heated and dried. The so obtained Powder is placed in a mold that is used to make a Disk at room temperature under a pressure of 100 N / mm² (1 t / cm²) is pressed for one to three minutes. The Slice is made using a crusher or Cutting unit roughly pulverized, through a sieve with a Given opening size of 300 microns and granulated. The Grains thus obtained are temporarily stored in two hours Air sintered at 1300 ° C, and the sintered grains are in turn passed through a sieve with an opening size of Given 300 microns. The coarse powder thus obtained is an aqueous solution of polyvinyl alcohol (PVA) and polyethylene glycol, PEG, added as a binder, stirred and then heated and dried. The powder that the binders added through a sieve with openings of 300 µm. The sieved powder is placed in a mold and added for one to three minutes Room temperature under a pressure of 30 to 50 N / mm² (300 to 500 kp / cm²) uniaxially to a substrate then pressed in air at one temperature for two hours of 1500 ° C is sintered. This way you get an anode substrate with a diameter of 130 mm and a thickness of 4 mm in the Ni-YSZ metal ceramic as an electrically conductive material in the porous matrix ZrO₂ (YSZ) stabilized with Y₂O₃.

Fig. 3 shows the crystal structure of an anode substrate made in the above manner, taken with a scanning electron microscope. The fine powder of the electrical conductors, which consists of a nickel-zirconium dioxide metal ceramic, is distributed and stored in the interstices of the matrix made of coarse zirconium dioxide powder. The nickel paths that cause the electrical conductivity in the anode substrate are cut off with the consequence of an increased specific resistance if the average grain size of the fine NiO-YSZ powder exceeds 10 μm. On the other hand, a porous matrix cannot be obtained when the fine grain is sintered if the average grain size of the fine NiO-YSZ powder is less than 0.1 µm. Therefore, the range of the grain size of the NiO-YSZ must be kept in the range of 0.1 µm to 10 µm.

Although in the example above the anode 6 is formed as a further layer over the anode substrate 7 , the anode 6 can also be omitted to simplify the manufacturing process since the anode substrate 7 already includes the functions of an anode. That is, the anode 6 is already formed within the anode substrate 7 . The manufacturing process continues with a YSZ electrolyte made of ZrO₂, which is stabilized with 8 mol% Y₂O₃, by plasma spraying directly onto the NiO-YSZ anode substrate obtained in the above manner in a thickness in the range of about 100 to about 200 microns is applied. Then a LaMnO₃ cathode layer is applied to the electrolyte. In this way, an electrolyte / electrode arrangement is obtained in which the anode substrate includes the functions of an anode.

Example 2

Magnesium oxide stabilized zirconium dioxide, MSZ, which consists of a coarse zirconium dioxide powder, partially stabilized with 9 mol% magnesium oxide, MgO, is temporarily sintered for about two hours at 1600 ° C, passed through a sieve with an opening size of 300 µm and granulated to an average grain size in the range between 50 and 100 microns. Using the same method as in Example 1, an anode substrate 7 with a diameter of 130 mm and a thickness of 3 mm is produced by incorporating a Ni-YSZ metal ceramic material as an electrically conductive material in the porous matrix consisting of MgO-stabilized ZrO₂ .

The mean linear coefficient of thermal expansion of nickel oxide, NiO, is about 15 × 10 ^-6 / ° C in air, from room temperature to about 1000 ° C, while the thermal expansion coefficient of MSZ is about 9 × 10 ^-6 / ° C in the same Temperature range. Therefore, the coefficient of thermal expansion of the anode substrate can be matched to the coefficient of thermal expansion of the yttria-stabilized zirconia, YSZ, of the solid electrolyte element of 10.5 × 10 ^-6 / ° C. by adding a suitable amount of MSZ to the NiO.

FIG. 4 shows a graphical representation of the relationship between the coefficient of thermal expansion and the amount of coarse YSZ or MSZ powder that is added to the anode substrate. While the thermal expansion coefficient of NiO-YSZ is 13 × 10 ^-6 / ° C without the addition of the coarse powder, it can be reduced to 11.4 × 10 ^-6 / ° C if 50% by mass of coarse YSZ powder is added will. On the other hand, the thermal expansion coefficient of the coarse MSZ powder can be matched to that of YSZ by adding 10 to 20% by mass of the coarse YSZ powder.

Fig. 5 shows in a graphical representation the relationship between the added amount of coarse YSZ and MSZ- powder to the anode substrate and the resistivity of the anode substrate. The increase in resistivity due to the addition of MSZ is slightly larger than that due to the addition of YSZ, but both YSZ and MSZ result in a resistivity of 50 mΩcm or less when the amount of YSZ or MSZ added is in the range up to 40 Mass% lies. The voltage drop during operation of the fuel cell due to this specific resistance is negligible. As long as the amount of coarse zirconia powder added is in the range of 5 to 40% by mass, thermal compatibility with YSZ can be achieved while maintaining the specific resistance at a low value.

Example 3

Nickel oxide, NiO, and zirconium oxide stabilized with yttrium oxide, YSZ, are in a weight ratio of 2: 1 weighed and to a solution of PVB, PEG and dioctyl phthalate as binders in a solvent mixture of 60% by mass of toluene and 40% by mass of isopropyl alcohol were added. The ingredients are used to prepare an electrically conductive Material wet mixed in a ball mill.

A porous matrix of zirconia felt is immersed in an aqueous solution of NiO-YSZ and then vented in a vacuum glove box for about 10 to 30 minutes under a pressure of approximately 21 kPa (160 mmHg). The matrix is then removed from the box, placed on a polyester film and then dried continuously in the glove box. The matrix is then sintered in air at a temperature of 1500 ° C. for about two hours, an anode substrate 7 having a diameter of 130 mm and a thickness of 3 mm being obtained. In this way, an anode substrate is created in which a NiO-YSZ metal ceramic material is embedded as an electrically conductive material in the porous matrix made of ZrO₂ stabilized with Y₂O₃. Instead of a felt, other materials can be used for the zirconium dioxide matrix, such as sponges, honeycombs, cloths or umbrellas. By introducing a zirconium dioxide matrix, the same specific resistance and the same thermal adaptation to YSZ as in example 2 are also possible in this example 3.

Although the electrolyte in the above examples the anode substrate is applied, exists within the Invention also allows the flat side of the cathode substrate to use as follows from the following Example 4 shows.

Example 4

A cathode substrate 10 is made in the following manner: lanthanum oxide, La₂O₃, and manganese carbonate, MnCO₃, are weighed in amounts sufficient to form LaMnO₃. To increase the electrical conductivity of LaMnO₃, strontium carbonate, SrCO₃, is added so that lanthanum manganite doped with strontium is obtained. The resulting mixture is temporarily sintered in air at a temperature of 1400 ° C for 3 hours. The product is pulverized in a ball mill for 24 hours. After it was determined by the X-ray diffraction method that the resulting powder has a perovskite structure, coarse YSZ powder is added as in Example 1, and polyvinyl alcohol, PVA, dissolved in water is further added as a binder. Then it is stirred and dried. The powder containing the binder is sieved by means of a sieve with an opening size of 300 µm to obtain a raw material powder. The raw material powder is put in a mold and uniaxially pressed for 1 to 3 minutes at normal temperature under a pressure of 50 to 100 N / mm² (500 to 1000 kp / cm²). The molded product is sintered for five hours in air at a temperature of about 1350 ° C. In this way, the cathode substrate 10 is obtained with a diameter of 130 mm and a thickness of 4 mm, in which an electrically conductive material made of lanthanum manganite is completely or partially stabilized in the porous matrix made of zirconium dioxide.

The mean linear thermal expansion coefficient of La _0.85 SrO _0.15 MnO₃ without the addition of coarse zirconium dioxide powder was 11.1 × 10 ^-6 / ° C in air at temperatures from room temperature to 1000 ° C. On the other hand, by adding 10 mol% of coarse YSZ powder, a cathode substrate 10 with a thermal expansion coefficient of 10.7 × 10 ^-6 / ° C. is obtained. Thereafter, YSZ plasma spraying was carried out at reduced pressure for the solid electrolytic element, and no warpage or cracking occurred in the substrate.

By inserting a zirconia matrix on the cathode side an electrolyte / electrode arrangement, which is thermally compatible with YSZ.

According to the invention, the electrically conductive material embedded in a porous matrix made up of the same Main components like the solid electrolyte element put together. Even if the electrically conductive Material physically through oxidation or reduction changes, the electrode substrate remains mechanical as a whole stable because the supporting element, i.e. the porous matrix, is stable. In addition, the electrical conductivity of the electrode substrate is increased in that the  Composition of the electrode substrate, which consists of a porous Matrix and electrically conductive material is optimized becomes thermally on while the electrode substrate adapted the solid electrolyte element in a unit cell is.

Claims (6)

1. Electrolyte / electrode arrangement for a solid electrolyte fuel cell, comprising:
a pair of flat electrodes spaced from each other;
a solid electrolyte element which is enclosed between the electrodes and contains at least one chemical component which imparts a certain coefficient of thermal expansion to the solid electrolyte element;
a porous matrix containing at least one chemical component that imparts the coefficient of thermal expansion of the solid electrolyte element to the porous matrix, one of the flat electrodes being formed on or within one side of the porous matrix, and
an electrically conductive material embedded in the porous matrix for imparting electrical conductivity to it in the direction of the thickness of the flat electrodes, wherein the solid electrolyte element and the porous matrix have chemical components which are substantially the same for causing the corresponding coefficients of thermal expansion. 2. Electrolyte / electrode arrangement according to claim 1, characterized in that the solid electrolyte element and the porous matrix contain zirconium dioxide, that at least partly by inclusion of yttrium oxide or Magnesium oxide is stabilized.   3. Electrolyte / electrode arrangement according to claim 2, characterized in that the at least partially stabilized zirconium dioxide has a grain size in Has a range of 50 to 100 microns. 4. Electrolyte / electrode arrangement according to one of claims 1 to 3, characterized characterized in that the porous matrix contains 5 to 40% by mass of zirconium dioxide. 5. Electrolyte / electrode arrangement according to one of claims 1 to 4, characterized characterized in that the electrically conductive material comprises a material from the group of nickel-zirconium dioxide-metal-ceramic material and lanthanum manganite. 6. Electrolyte / electrode arrangement according to claim 5, characterized in that the nickel-zirconia metal-ceramic material or the lanthanum manganite has an average grain size in the range of 0.1 to 10 microns.